retrieving mail from an external server

Now that I have zimbra running and stable I want to plan how I am going to get mail. Here is what I have to work with....
I have a school with about 30 users that use an outside email server for mail and webmail. There is no spam or virus protection so they get at least 40-60 spams per user per day. I want to setup an internal mail server (zimbra) that can retrieve mail from the outside source, filter it, and dispense it.
I was thinking I would setup 1 user on the outside server that is catchall and then retrieve that mail fia fetchmail and then disperse it to the users from zimbra. Is that the best way to handle it and can I do it that way or is there a better way?
I want to be sure I am researching along the right lines of thinking.....
any ideas?
thanks
Bill

That was my first thought but the school gets comcast for free and has a dynamic ip.... they dont want to pay for a static ip. I was concerned that a dynamic ip address would not be a stable setup.... your thoughts?

Using a Dynamic IP for receiving email isn't really a problem but sending _may_ be. You may end up on blacklists and certainly AOL doesn't accept mail from servers on a dynamic IP (or from ISPs that supply domestic users - I don't know if you fall into that category). You also have another problem in that the mailbox you're retrieving your mail from may not be suitable. You really need a multi-drop mailbox and almost no ISPs provide that, it's just as you mentioned earlier - a catch-all mailbox. Have a look at this article on why multi-drop is not a good idea.

The most effective way I could think of doing this (and it may be a pig to administer) is to use fetchmail to drop all the mail as-is into a local mailbox and then use filters to distribute it to your users, that way it would go through the local Zimbra anti-spam system.

yes, I already found that using a dyn address on my test box here at home has that problem. even my own isp wont allow emails from my home system to go thru. it blacklists the address. I am afraid the same thing would happen at the school. The external server does have a catch all I could setup so I could retrieve all mail from it. SO, would I then use filters to forward to the users locally? The users now send thru the comcast isp so sending shouldnt be a problem.... I dont think.
SO, if I could get a static ip to use, and then setup zimbra as the first server and have the external as a backup than when the school loses its connection, ( happens about once a month) then the external could collect email.